Olefinic containing monomers can be polymerized using organo-alkali metal initiators, such as butyllithium. The resultant intermediate polymer contains an active alkali metal end group, which can be subsequently reacted with a suitable protonating, functionalizing, or coupling or linking agent to replace the alkali metal with a more stable end group. In many applications, it can be useful to react the polymer living end with a functionalizing agent, such as ethylene oxide, to provide a polymer having a terminal functional group, such as a hydroxyl, carboxyl, or amine group.
Telechelic polymers contain two functional groups per molecule at the termini of the polymer and are useful in a variety of applications. For example, telechelic polymers have been employed as rocket fuel binders, in coatings and sealants, and in adhesives. One approach that has been used to prepare telechelic polymers is the generation and subsequent functionalization of a “dilithium initiator.” A dilithium initiator can be prepared by the addition of two equivalents of secondary butyllithium to meta-diisopropenylbenzene. The dilithium initiator is then reacted with suitable monomers, such as butadiene, to form a polymer chain with two anionic sites. The resultant polymer chain is then reacted with two equivalents of a functionalizing agent such as ethylene oxide.
While useful, gelation is frequently observed during the functionalization step. This leads to lower capping efficiencies (see, for example, U.S. Pat. No. 5,393,843, Example 1, wherein the capping efficiency was only 82%). Additional details of this gelation phenomena are described in U.S. Pat. No. 5,478,899. Further, this dilithium approach can only afford telechelic polymers with the same functional group on each end of the polymer chain.
Progress has been made in the synthesis of dihydroxy terminated polymers. For example, monofunctional silyl ether initiators containing alkali metal end groups are disclosed in GB 2,241,239. These monofunctional silyl ether initiators were demonstrated to be useful in producing polybutadienes having an alpha protected hydroxyl functional group. The living polymer can be reacted with suitable functionalizing agent such as ethylene oxide, and the silyl protecting group removed to provide a dihydroxy telechelic polymer.
Monofunctional ether initiators of the formula M—Z—O—C(R1R2R3) wherein M is an alkali metal, Z is a branched or straight chain hydrocarbon tether group, and R1, R2 and R3 are independently defined as hydrogen, alkyl, substituted alkyl, aryl or substituted aryl, have also been proposed as anionic polymerization initiators to introduce a protected hydroxyl functionality into a polymer. See U.S. Pat. No. 5,621,149. The hydrocarbon solubility of such initiators can be increased by chain extension of the initiator with a conjugated diene. See U.S. Pat. No. 5,565,526.
Anionic initiators containing a tertiary amine functionality have also been proposed for use in hydrocarbon solvent polymerizations. Such initiators have the general formulaM—Z—N—(C—R1R2R3)2wherein M is defined as an alkali metal selected from lithium, sodium and potassium; Z is defined as a branched or straight chain hydrocarbon connecting group which contains 3-25 carbon atoms; and R1, R2 and R3 are independently defined as hydrogen, alkyl, substituted alkyl groups, aryl or substituted aryl groups. See M. J. Stewart, N. Shepherd, and D. M. Service, Brit. Polym. J., 22, 319-325 (1990).
However, these amine functional initiators possess low solubility in hydrocarbon solvents (typically less than 0.3 Molar in aliphatic or cycloaliphatic solvents like hexane or cyclohexane). The addition of an ethereal co-solvent does increase the solubility of these initiators; however, this also increases the amount of 1,2-microstructure in the resultant polymer. See H. L. Hsieh and R. P. Quirk, Anionic Polymerization Principles and Practical Applications, pp. 397-400. Various other techniques have been employed to increase the solubility of these initiators in hydrocarbon solvent. For example, chain extension of the initiator with a conjugated diene increased the solubility several fold. See U.S. Pat. No. 5,527,753.
The synthesis of diamino terminated polymers remains relatively unexplored. Nakahama reports the preparation of amino terminated polystyrene by trapping the dianion with an electrophile that contained a protected amine group. A high degree of functionality was achieved by this technique. See K. Ueda, A. Hirao, and S. Nakahama, Macromolecules, 23, 939-945 (1990). However, the reaction conditions (−78° C., THF solvent) were not practical for commercial production of these functionalized polymers.
El-Aasser et al. recently reported the preparation of amino terminated telechelic polybutadiene by a free radical approach. See J. Xu, V. L. Dimonie, E. D. Sudol, and M. S. El-Aasser, Journal of Polymer Science: Part A: Polymer Chemistry, 33, 1353-1359 (1995). Since this is a free radical synthesis, little control of molecular weight, molecular weight distribution, and position of the amine functional group was obtained.